1. Field of the Invention
The present invention relates to a switching power supply adopting a synchronous rectification method, and more particularly to a step-up switching power supply that achieves an improvement in the power conversion efficiency by reducing the loss at the rectifying device.
2. Discussion of Background
Switching power supplies, achieving a high degree of power conversion efficiency and can be constituted as compact units, are adopted in a wide range of applications - as power sources in various types of industrial apparatuses and consumer apparatuses including computers and office automation apparatuses. At present, as battery driven apparatuses such as portable information terminals and communication apparatuses are used by a greater than ever number of people, step - up switching power supplies need to achieve a further improvement in the efficiency.
As a means for meeting this requirement, switching power supplies adopting a synchronous rectification method have been proposed. Such switching power supplies adopt a synchronous rectification method and are developed based upon the principle that the saturation voltage between the collector and the emitter of a transistor in an ON state is lower than the forward descending voltage at a diode. The diode is normally used as a rectifying device in a rectifier circuit in the prior art. The switching power supply achieves an improvement in the switching source efficiency by replacing a diode with a transistor to reduce the power loss occurring at the rectifying device.
An example of a switching power supply adopting the synchronous rectification method is disclosed in Japanese Unexamined Patent Publication No. 289534/1996. The switching power supply disclosed in this publication is a step-up/step-down switching power supply adopting the synchronous rectification method. The switching power supply is provided with a choke coil to which a DC input voltage is applied, a switching element that cuts off the voltage intermittently, and a rectifier/smoothing circuit that rectifies and smooths the switching output. A synchronous rectifying device at the rectifier/smoothing circuit is constituted of a bipolar PNP transistor. The voltage manifesting at the two ends of the choke coil is used as a drive signal for the PNP transistor. Thus, compared to switching power supplies that employ a diode as the rectifying device, the power loss can be reduced. Furthermore, since the voltage manifesting at the two ends of the choke coil is directly utilized as a drive signal for the PNP transistor, an advantage is achieved in that the drive circuit is realized through a simple structure.
However, the saturation voltage between the collector and the emitter of a bipolar PNP transistor is high, at approximately 0.2V compared to the saturation voltage in a bipolar NPN transistor which is normally approximately 0.1V. Thus, it is difficult to make the most of the advantages of the bipolar transistor in a switching power supply adopting the synchronous rectification method that employs a PNP transistor for a current control element.
In addition, a bipolar high frequency power transistor, which is ideal in applications in this type of switching power supply, is constituted of silicon, and the relative difficulty in manufacturing PNP silicon transistors compared to NPN silicon transistors presents a stumbling block in achieving an inexpensive switching power supply.